From related art, in industrial vehicles and the like, a fluid control device equipped with a plurality of switching valves has been known. This device is used by connecting an actuator to each switching valve, respectively. As an actuator connected to the switching valve of such a fluid control device, for example, a lift cylinder for raising and lowering a loading platform in a forklift or a tilt cylinder for inclining a mast supporting the loading platform forward and backward is adopted.
Incidentally, in such a fluid control device, in some cases, hydraulic fluid pressures necessary for operating each actuator may be different from each other. In other words, the hydraulic fluid pressure necessary for raising the loading platform, the hydraulic fluid pressure necessary for inclining the mast forward or backward, and the hydraulic fluid pressure necessary for an actuator for an attachment such as a clamping function which is a case of being added separately are different from each other. From this, it is conceivable to provide a plurality of relief valves, such as providing the relief valves at each cylinder port. Specifically, it is conceivable to provide a main-relief valve for preventing the hydraulic fluid pressure in a passage for supplying the hydraulic fluid from a fluid pressure supply source to each actuator from exceeding a predetermined first fluid pressure which is a highest fluid pressure, and it is conceivable to provide a sub-relief valve for preventing the hydraulic fluid pressure supplied to an actuator for other tilt or attachment from exceeding a second fluid pressure lower than the first fluid pressure, with respect to an actuator such as a lift cylinder requiring the highest hydraulic fluid pressure. When such a configuration is adopted, an actuator which does not require a high hydraulic fluid pressure and a pipe associated with the actuator are protected. In addition, by adopting the actuator and the pipe having relatively low pressure resistance performance, it is possible to reduce the cost. Installing examples of the sub-relief valve are as follows (see, for example, Patent Documents 1 and 2).
In Patent Document 1, when the switching valve connected to the lift cylinder, which is an actuator requiring the highest hydraulic fluid pressure, takes a position other than a raised position to direct the hydraulic fluid to the lift cylinder, the hydraulic fluid from the hydraulic pressure supply source is directed to the sub-relief valve. However, when this configuration is adopted, the following problems may occur. That is, when another actuator is operated in a state in which the lift cylinder is located at the raised position, since the hydraulic fluid is not directed to the sub-relief valve, the hydraulic fluid at a higher pressure than the second fluid pressure may be directed to the another actuator. At this time, another actuator or the pipe associated with the actuator may be broken unless devices capable of withstanding the highest pressure are selected. On the other hand, when another actuator or the pipe is made to withstand the high pressure, although it is possible to prevent breakage, another problem arises in which it is not possible to achieve the cost reduction as described in the preceding paragraph.
In Patent Document 2, when a switching valve connected to a member other than the lift cylinder takes a position for operating the actuator, that is, a position other than a neutral position, the hydraulic fluid from the hydraulic pressure supply source is directed to a sub-relief passage including a sub-relief valve, other than the actuator. The sub-relief passage branches inside the switching valve. When such a configuration is adopted, even in a case where the lift cylinder is located at the raised position, the sub-relief valve operates when the hydraulic fluid pressure directed to another actuator exceeds the second fluid pressure. Thus, the problem described in the previous paragraph is solved. However, when adopting such a configuration, in order to prevent the occurrence of problems due to the fact that the actuators communicate with each other inside the switching valve and via the sub-relief passage in a case where the plurality of switching valves simultaneously take the positions other than the neutral position, it is necessary to separately provide a check valve in the sub-relief passage. Therefore, there is another problem of increases in the number of components and assembling man-hours.